In a modern thermal ink-jet printhead device, resistors used to propel bubbles of ink toward the paper substrate must be able to fire for millions of times without failure. In thermal printhead printing, the purity and chemical structure of the dye in the ink can radically affect the printhead operation, resulting in resistor failure or nozzle clogging.
Changing the cation of the dye molecule improves the solubility of the dye molecule in the solvent, which prevents crusting (clogging of the printhead nozzle from dye precipitation during storage). Selection of the proper cation can also significantly improve resistor life of the printhead. Substitution of FD&C Blue Dye #1 improves resistor life from about 13 million firings to about 25 million firings by changing the cation associated with the dye from sodium to triethanol amine.
Other cations have also been substituted for sodium in an attempt to reduce crusting; examples include lithium and tetramethyl ammonium cations. In the case of thermal ink-jet printers, such substituted cations must not result in an ink that evidences a worsened kogation.
It is known that sodium-containing dyes suffer from crusting. Yet, these dyes often have better kogation properties than many of the cation-substituted dyes. Kogation is a coined term, unique to thermal ink-jet printing, which describes the extent of decomposition of the ink on the resistors of thermal ink-jet printers as a consequence of resistive heating.
Often, a balance between the desirable low crusting properties of the cation-substituted dye and the desirable low kogation properties of the sodium-containing dye may be achieved by blending the cation-exchanged dye with sodium-containing dye, the latter possibly purified by the reverse osmosis process described above to remove excess sodium.
In formulating such a blend, many factors will ordinarily be considered, such as the construction of the ink pen, that is, the resistor design, how hot the resistors are fired, the character of the surface (the extent of carbon sticking from the decomposition of the ink, or kogation), the thickness of layers for heat transfer, and so forth.
Copper phthalocyanine dyes find extensive use in thermal ink-jet printing. However, the sodium-containing dyes (which are commercially available) tend to form plugs (both hard and soft) in the orifices of the printheads when the printer is idle. Such plugs may be difficult to completely blow away before the printhead can be used again. This operation of blowing away is called repriming, and must be done prior to use of the pen. Algorithms in the printer are used to control the number of times of repriming. Of course, ink is lost in repriming, and thus dyes with reduced plug-forming ability would reduce the frequency of repriming. Finally, some cationic forms yield plugs which cannot be removed simply by priming and thus render the pen unusable; examples of such cationic forms include potassium-Food Black 2 and sodium-Acid Red 87.